Charlotte's nice. That's about all we know. She takes no risks on anyone's behalf, thank you very much, and she's all about self-preservation.
Savanna's a frenemy/mean girl-in-the-making. She invites Summer to her Halloween party, which Summer thinks is really cool… until she walks in to the party and the first (facetious) words out of Savanna's mouth are, "'Where's your boyfriend, Summer?'" (3.The Halloween Party.4). Savanna's intention is to recruit Summer into their group, but for some reason she thinks the best way to go about doing so is to make fun of Summer for her friendship with Auggie. Needless to say, it doesn't work.